The invention generally relates to continuous surface wave analysis for three-dimensional seismic data.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying seismic source(s) and seismic sensors at predetermined locations. The sources generate seismic waves, which propagate into the geological formations creating pressure changes and vibrations along their way. Changes in elastic properties of the geological formation reflect, refract or scatter the seismic waves, changing their direction of propagation and other properties. Part of the energy emitted by the sources reaches the seismic sensors. Some seismic sensors are sensitive to pressure changes (hydrophones), others to particle motion (e.g., geophones), and industrial surveys may deploy only one type of sensors or both. In response to the detected seismic events, the sensors generate electrical signals to produce seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
For a land-based seismic survey, the seismic waves include surface waves and body waves. The surface waves propagate without radiation into the Earth's interior, are parallel to the Earth's surface and have a reduced geometric spreading as compared to the body waves. The surface waves carry a significant part of the energy that is radiated by a seismic source at the Earth's surface.
Surface waves constitute most of the coherent noise in seismic data. In this manner, the surface waves are source-generated events that are characterized by relatively low velocity and relatively high amplitudes, and the surface waves superimpose onto the useful signal. This coherent noise (often called ground roll in land seismic applications) may be in the form of many different wave types, such as Rayleigh waves with multiple modes of propagation, Lamb waves, P-guided waves, Love waves and Scholte waves.
The propagation properties of surface waves depend on the elastic properties of the “near-surface,” the shallow portion of the Earth, which is responsible for most of the perturbation and degradation of the acquired seismic data. For purposes of designing filters to attenuate surface wave noise, it is generally important to accurately identity the properties of the surface waves. Additionally, knowledge of the surface wave properties may be beneficial for other purposes, such as determining the local elastic properties of the near surface and estimating static corrections.